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EVAPORATION                                        375
ing system will have considerable more air leakage than a standard engine or turbine. Therefore the condenser and vacuum pump should always be very much larger than is usually figured for standard steam practice.
Preheaters.—Under "Steam Consumption/' it has been shown that the temperature of the incoming liquor has considerable influence on the steam consumption of the plant. Wherever possible, waste steam or waste gases should be used to preheat the cold liquors, so that they will enter the evaporator at about the boiling temperature in the first effect. For larger installations it is advisable to place preheaters in all the vapor lines of the multiple effect, and pass the liquor from one heater to the other, utilizing the vapors coming from each effect. This would mean that the liquor is preheated in multiple effect, and larger plants will show a considerable saving by this method of operation.
Separators and Catchalls.—To prevent losses by entrainment or foaming, it is necessary to install efficient separators and catchalls. For ordinary conditions, standard so-called "oil separators" with baffle plates may be usM, but for extreme cases, separators of the centrifugal type, or very large expansion tanks with baffles must be installed. Prof. E. W. Kerr has given the results of some tests made with a Swartwout separator, which has shown extremely high efficiency (See Bull. 138 and 149 of the Louisiana State University).
Specific Uses of Evaporators.—The following pages will give in convenient form the information and data that are necessary for the design or selection of a suitable evaporator for various kinds of solutions. The figures for capacity are given from practical experience under ordinary working conditions, and may be exceeded in many cases, especially in new plants where the heating surface and all joints are in first class condition.
Distilled Water.—Water will boil with a small temperature difference, and the apparatus is simple and cheap. It is therefore economical to combine a number of effects to one unit, and quadruple- and decituple-effect evaporators of the vertical-tube and film type are common practice. They are operated under pressure and vacuum, and the capacity will vary from 3 to 6 gal. per square foot according to the total temperature difference. Evaporator shells are made of cast iron or steel, and tubes of steel, brass or copper. There is no foaming, but generally all raw water contains considerable amounts of scale forming material and tubes have to be cleaned frequently. Sea water has to be used to a considerable extent for the manufacture of distilled water, and usually the Reilly coil type is used aboard ships, as it is claimed by the inventor that scale will not deposit on the tubes. Sometimes the 20°B6. liquor coming from the sea water stills is concentrated further for the recovery of sea salt in standard vertical-tube evaporators with salt filters.
Ammonium Chloride.—The weak liquor will contain from 6 per cent to 10 per cent of salt, and is usually concentrated to 40 per cent in a horizontal-tube or vertical-tube evaporator. Capacity is about 2 gal. per square foot with a